Die-casting brass alloy which is resistant to dezincification

ABSTRACT

The present invention relates to a die-casting brass alloy having a dezincification resistance, which is lower than 100 μm for a separate value according to British Standard BS 2872 in a die-casting condition (i.e. without a subsequent phase transforming heat treatment). The alloy according to the invention is characterized by the following composition: 
     
       
         
               
               
               
               
             
                   
                   
               
                   
                 Cu: 
                 63.6 
                 weight-% 
               
                   
                 Pb: 
                 1.8 
                 weight-% 
               
                   
                 Si: 
                 0.73 
                 weight-% 
               
                   
                 Al: 
                 0.07 
                 weight-% 
               
                   
                 As: 
                 0.06 
                 weight-% 
               
                   
                 Ni: 
                 0.2 
                 weight-% 
               
                   
                 Sn: 
                 0.3 
                 weight-% 
               
                   
                 Fe: 
                 0.25 
                 weight-% 
               
                   
                 B: 
                 8 
                 ppm 
               
                   
                 Other impurities: 
                 max. 0.3 
                 weight-% 
               
               
               
               
             
                   
                 Zn: 
                 remainder.

FIELD OF THE INVENTION

The present invention relates to a die-casting brass alloy, which isresistant to dezincification.

BACKGROUND OF THE INVENTION

Dezincification is a problem for brass water fittings, when the waterquality varies and maybe is strongly corrosive.

It is known, that it is possible to treat the copper rich alpha-phase inbrass against dezincification by means of small additions of arsenic orantimony, whereas the zinc rich beta-phase is not resistant todezincification.

Thus, it would be logical to keep a high percentage of copper in a brassalloy resistant to dezincification (as an alloy 1 in FIG. 1, showing aportion of the phase diagram Cu—Zn, Hansen, Constitution of binaryalloys, New York 1958) in order to minimize or completely avoid theamount of the less corrosive resistant beta-phase. The problem with suchan alloy is, that it results in a primary solidification of thealpha-phase in the form of long solidification crystals, so calleddendrites, which means, that the beta-phase will form long bands betweenthe alpha-dendrites. This results in two negative consequences:

a) The material will be brittle by heat; and

b) The material will obtain a deep dezincification, since thedezincification will follow the long beta-phase bands.

This phenomenon is thoroughly described in the following scientificarticle: Arno Louvo, Tapio Rantala, Veijo Tauta, “The Effect ofComposition on as-cast Microstructure of alfa/beta-Brass and its Controlby Microcomputer”, LISBOA 84, 51 st International Foundry Congress.

FIG. 2, which has been excerpted from this article, describes theproblem with brittleness by heat, and FIG. 3, which has been excerptedfrom the same article, the phenomenon with increasing dezincificationdepths with an increasing copper content.

In order to avoid the above-mentioned problems the alloy must solidifyprimarily in the beta-phase as an alloy 2 in FIG. 1, which allows thefollowing advantages:

a) The amounts of micro and macro segregations will be substantiallylower for an alloy, which solidifies primarily in the beta-phase. Thisis caused by the fact, that the diffusion speed in the beta-phase isabout 1000 times higher than in the alpha-phase, which is a result ofthe fact, that its crystal structure has an atom arrangement accordingto bcc (body-centered-cubic) as compared to the atom arrangement of thealpha-phase fcc (face-centered-cubic).

b) The solidification crystals may be fine grain-treated with boron,which forms fine grains in a very efficient way, and only extremelysmall amounts of this substance is needed to obtain a fine grain-formingeffect. According to experience boron does not have a fine grain-formingeffect on brass, which solidifies primarily in the alpha-phase, whereasit is very efficient as far as nucleation of beta-crystals is concerned.

The drawback is, that the beta-phase amount increases in the finalcasting structure and without a heat treatment it will be difficult tomeet the toughest dezincification requirements, which requires a maximaldezincification depth of 100 μm as a separate value. This is true aboveall for heavy thicknesses of material, shown in FIG. 3.

The information above are known basic facts.

Additional already known techniques are described in WO 89/08725 A1, EP0 572 959 A1 and MNC manual no. 8, edition 2, September 1987,“Specialmässing”, page 43.

The object of the present invention is to suggest a way of eliminatingthe above-mentioned drawbacks.

SUMMARY OF THE INVENTION

This object is attained according to the invention by the development ofan alloy having the following characteristics.

By balancing copper, zinc, silicon and aluminum in a capable manner itis possible to attain a solidification in the beta-phase andnevertheless avoid the development of continuous beta-phase areas in thefinished product. The beta-phase will be found in isolated agglomeratesin a matrix of alpha-phase, which is protected against a dezincificationdue to the arsenic addition. The primary solidification in thebeta-phase with the alloy combination according to the inventioncombined with the high solidification speed of the die-casting limitsthe size of the agglomerates of the beta-phase in the final castingstructure, the agglomerates also in a thick diecasting material with alow solidification speed obtaining an extension, which is clearly lessthan 100 μm. By means of fine grain-treatment with boron the size of theagglomerates and consequently also the depth of the dezincification canbe additionally reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example, with referenceto the accompanying drawings in which:

FIG. 1 shows a portion of a phase diagram Cu—Zn;

FIG. 2 describes a problem with brittleness by heat;

FIG. 3 shows a phenomenon with increasing dezincification depths with anincreasing copper content;

FIG. 4 shows how the amount of peritecticly solidifying materials(solidification primarily in the alpha-phase) quickly is reduced, whenthe copper content in the alloy is reduced, whereas the increase of theamount in the beta-phase in the final structure increases relativelyslowly;

FIG. 5 shows the result from investigations of the dezincification depthaccording to the international standard ISO 6509 for die-cast workpieces having a 6 mm thickness of material as to alloys having a varyingCu content; and

FIG. 6 shows the result for the corresponding investigation with amaterial thickness of 16 mm.

DETAILED DESCRIPTION OF THE INVENTION

These conclusions have been confirmed by the results of an extensivedevelopment effort during several years, the purpose of which has beento find appropriate alloy combinations.

FIG. 4 shows how the amount of peritecticly solidifying materials(solidification primarily in the alpha-phase) quickly is reduced, whenthe copper content in the alloy is reduced, whereas the increase of theamount in the beta-phase in the final structure increases relativelyslowly.

FIG. 5 shows the result from investigations of the dezincification depthaccording to the international standard ISO 6509 for die-cast workpieces having a 6 mm thickness of material as to alloys having a varyingCu content. The result is unambiguous. A dezincification minimum isattained exactly in the area, where the peritecticly solidificationceases, at the same time as the amount of beta-phase has not yet becometoo large. The figure shows a dezincification depth for a maximalseparate value as well as median values for a number of measurements,done on the same test object. The result is, that in a relatively widearea the obtained result falls below the requirements regarding thedezincification resistance of maximally 100 μm for a separate value.

The object of the invention is to suggest an alloy, which also meets thedezincification requirements for thick die-cast materials, and FIG. 6shows the result for the corresponding investigation with a materialthickness of 16 mm. Also for this material thickness the requirement ismet, namely maximally 100 μm for a separate value but within a morenarrow interval.

At a Cu content of lower than 63.6% the beta-phase agglomerates becomeso large, that they start to grow together, which results in a too largedezincification.

At a Cu content of higher than 64.1% the amount of primarysolidification in the alpha-phase becomes so large, that long beta-phasebands develop between the alpha crystals and consequently a deepdezincification is obtained.

The positive results of this balancing of the alloy ingredients aresummarized as follows:

1) Die-cast material, made of the alloy, meet, without a subsequent heattreatment, the requirements as to a maximal dezincification depth of 100μm for a separate value

2) The alloy can be fine grain-treated with boron in an efficient way,which results in a most fine-grained structure in the finished product,which results in two advantages:

The dezincification resistance is further improved, because the size ofthe beta-phase agglomerates is further reduced; and

The porosity in the die-cast material is distributed more evenly and theseparate size becomes smaller, which reduces the risk of a leakydie-cast material and consequently the rejection costs for products,which must meet pressure impenetrability requirements, are also reduced.

3) The aluminum content can be kept at a low level, 0.03-0.1 weight-%,which means, that the positive effect of the aluminum addition on adie-casting alloy is utilized, but the negative effects are avoided.

Positive effects include the strong dezincification effect of aluminum,which means, that also at a low aluminum content the oxygen content inthe melt is stable and very low. Aluminum exerts also in small amounts apurification effect in such a way, that it reduces a zinc oxide coatingon pouring cups, molding tools and cores; and

Negative effects include the formation in alloys, which include siliconand in which the aluminum content is larger than 0.1 weight-%, of asticky slag, which consists of aluminum silicates. When a melt isapplied with a cup, a portion of this slag will be introduced into theproduct, in which it forms “hazes” and “balls”. These inclusions impairthe mechanical properties of the finished product, but, what is worse,they function as capillaries, which means, that the dezincificationfollows the inclusions, if they reach the surface, which results in deepdezincifications, which by far goes beyond the requirements regardingthe dezincification resistance of a maximally 100 μm dezincification fora separate value.

In this respect the present invention differs from the finegrain-treated alloy according to DE-A 43 18 377 A1, which recommends analuminum content of 0.3-0.7 weight-% and a silicon content of 0.3-0.7weight-%.

A dezincification resistant alloy according to the present invention ischaracterized by the following compositions:

Cu: 63.0-65.5 weight-% Pb: 1.5-2.2 weight-% Si: 0.6-0.9 weight-% Al:0.03-0.1 weight-% As: 0.03-0.1 weight-% Ni: max 0.5 weight-% Sn: max 0.5weight-% Fe: 0.1-0.5 weight-% B: 0-15 ppm Other impurities: max. 0.3weight-% Zn: remainder

An example of a specified alloy, which has been produced for quite along time on a large scale, has turned out to meet the requirementsaccording to the invention quite well:

Cu: 63.6 weight-% Pb: 1.8 weight-% Si: 0.73 weight-% Al: 0.07 weight-%As: 0.06 weight-% Ni: 0.2 weight-% Sn: 0.3 weight-% Fe: 0.25 weight-% B:8 ppm Other impurities: max. 0.3 weight-% Zn: remainder

The invention is not limited to the preferred embodiments specifiedabove, but it can be modified and supplemented in an arbitrary fashionwithin the scope of the inventive idea and the following claims. This isparticularly true, as regards the lead content, since lead is notdissolved in the alloy but remains as a separate phase, which does notinfluence the dezincification resistance. This means, that, if the leadcontent is reduced to below the specified interval, the rest of thealloy elements must be adjusted stochiometrically.

What is claimed is:
 1. A die-casting brass alloy resistant todezincification, the alloy comprising a composition of: Cu: 63.6-65.0weight-% Pb: 1.8-2.2 weight-% Si: 0.6-0.9 weight-% Al: 0.03-0.1 weight-%As: 0.03-0.1 weight-% Ni: 0.2-less than 0.5 weight-% Sn: max 0.5weight-% Fe: 0.1-0.5 weight-% B: 8-less than 15 ppm Other impurities:max. 0.3 weight-% Zn: remainder.


2. A diecasting brass alloy having a dezincification resistance lowerthan 100 μm, characterized by the following composition: Cu: 63.6weight-% Pb: 1.8 weight-% Si: 0.73 weight-% Al: 0.07 weight-% As: 0.06weight-% Ni: 0.2 weight-% Sn: 0.3 weight-% Fe: 0.25 weight-% B: 8 ppmOther impurities: max. 0.3 weight-% Zn: remainder.